1. Field of the Invention
The present invention relates to a switching power supply apparatus, and to an electric appliance incorporating it. More particularly, the present invention relates to a switching power supply apparatus provided with an overcurrent protection function whereby the output current is limited in an overloaded condition, and to an electric appliance incorporating such a switching power supply apparatus.
2. Description of Related Art
There has invariably been a strong demand for size and cost reduction in switching power supply apparatuses and in electric appliances incorporating them. In response, in switching power supply apparatuses, size and cost reduction has been attempted and achieved by reducing the inductances and capacitances of coils and capacitors. To reduce the inductances and capacitances of coils and capacitors, it is essential to increase the switching frequency. Thus, the switching frequency, which has conventionally been about several tens of kHz to 100 kHz, is now typically as high as about 1 MHz to 4 MHz.
In switching power supply apparatuses of the so-called current-control type, i.e., those that determine the on/off ratio (duty) of the switching device by comparing the output voltage with the switch current flowing through the switching device, it is known that a duty higher than 50% provokes unstable operation, causing low-frequency oscillation. When low-frequency oscillation occurs, the switching frequency usually drops to a fraction of its normal frequency, producing undesirable effects including increased ripples in the output voltage. To avoid this, in a current-control-type switching power supply apparatus, as shown in FIG. 6, it is customary to provide a current compensation circuit to ensure stable operation even with a duty higher than 50%.
FIG. 6 is a circuit block diagram showing the electrical configuration of a conventional switching power supply apparatus. In FIG. 6, a direct-current voltage fed from an unillustrated direct-current source is fed in via an input terminal IN. An input capacitor C1 for smoothing is connected between the input terminal IN and ground. The input terminal IN is connected through a current control portion 1 to one end of a switching device 2 such as a transistor, and the other end of the switching device 2 is connected to the cathode of a diode (rectifying device) D1 and to one end of a coil L1. The anode of the diode D1 is connected to ground. The other end of the coil L1 is connected to an output terminal OUT, and is also connected through an output capacitor C2 to ground. The other end of the coil L1 is also connected through serially connected voltage division resistors R1 and R2 to ground. Outside the switching power supply apparatus, a load 9 is connected between the output terminal OUT and ground.
The node between the voltage division resistors R1 and R2 is connected to the inverting input terminal (−) of a differential amplifier 3, and the non-inverting input terminal (+) of the differential amplifier 3 is connected to a reference voltage source 4. The output terminal of the differential amplifier 3 is connected to the inverting input terminal (−) of a comparator (comparison portion) 5, and the non-inverting input terminal (+) of the comparator 5 is connected to the current control portion 1. The output terminal of the comparator 5 is connected to the reset input terminal R of a flip-flop (switching control portion) 6, and the set input terminal S of the flip-flop 6 is connected to an oscillator (oscillation portion) 7. The output from the output terminal Q of the flip-flop 6 is fed, as a control signal, through a drive circuit 8 to the switching device 2. Thus, as the flip-flop 6 toggles between a set and a reset state, the switching device 2 turns on and off.
The current control portion 1 includes a current detection circuit (current detection portion) 11, an overcurrent detection circuit (overcurrent detection portion) 12, and a current compensation circuit 13. The current detection circuit 11 is connected between the input terminal IN and the switching device 2. The current detection circuit 11 monitors the switch current Isw that flows through the switching device 2, and outputs a detection current (detection signal) Isens that varies in proportion to the switch current Isw. The overcurrent detection circuit 12 compares the detection current Isens from the current detection circuit 11 with a set current (predetermined threshold value) Iocp previously set to correspond to the overcurrent detection level, and detects whether or not the switch current Isw is an overcurrent. The current compensation circuit 13 supplies a current for current compensation. The compensation current (first current compensation signal) from the current compensation circuit 13 is added to the detection current Isens from the current detection circuit 11, so that a compensated current Icc1 (first current-compensated signal) is fed to the non-inverting input terminal (+) of the comparator 5.
Next, the operation of this switching power supply apparatus configured as shown in FIG. 6 will be described. The direct-current voltage that is fed in via the input terminal IN is smoothed by the input capacitor C1 to become an input voltage Vin, which is then converted into a pulsating voltage by the switching operation of the switching device 2. When the switching device 2 is on, a current flows from the input terminal IN to the coil L1. This causes energy to be accumulated in the coil L1, and also causes energy to be fed to the load 9. On the other hand, when the switching device 2 is off, the energy accumulated in the coil L1 is fed through the diode D1 to the load 9. An output voltage Vo smoothed by the output capacitor C2 appears at the output terminal OUT, and this output voltage Vo is applied to the load 9, causing a load current Io to flow through the load 9.
The switching device 2 turns on and off as the flip-flop 6 toggles between different states. Specifically, through the drive circuit 8, the switching device 2 is so controlled as to turn on and off when the flip-flop 6 is in the set and reset states, respectively. FIGS. 7A to 7F are waveform diagrams in explanation of the operation of the switching power supply apparatus shown in FIG. 6. FIG. 7A shows the waveform of the pulse signal outputted from the oscillator 7. FIG. 7B shows the waveform of the output signal from the output terminal Q of the flip-flop 6 that turns the switching device 2 on and off. FIG. 7C shows the waveform of the switch current Isw in the normal condition. FIG. 7D shows the waveforms of the detection current Isens, the compensated current Icc1, and the error current (error signal) Ie in the normal condition. FIG. 7E shows the waveform of the switch current Isw in the overcurrent-protection-enabled condition. FIG. 7F shows the waveforms of the detection current Isens and the set current Iocp in the overcurrent-protection-enabled condition.
In FIGS. 7D and 7F, the values of the detection current Isens as observed at the moment that the switching device 2 turns from on to off and at the moment that it turns from off back to on are connected together by broken lines.
Receiving the pulse signal (FIG. 7A) from the oscillator 7, the flip-flop 6 is set at the trailing edges of the pulse signal, turning the switching device 2 on (FIG. 7B). On the other hand, the flip-flop 6 is reset when the signal from the comparator 5 turns to H (high) level, turning the switching device 2 off (FIG. 7B). As the switching device 2 turns on and off, a switch current Isw as shown in FIG. 7C flows through the switching device 2. Here, the control whereby the switching device 2 is turned off is achieved in the following manner.
As shown in FIG. 7D, the comparator 5 compares the compensated current Icc1, which is the sum of the detection current Isens from the current detection circuit 11 and the compensation current from the current compensation circuit 13, with the error current Ie from the differential amplifier 3. If the compensated current Icc1 is larger, the comparator 5 turns its output signal to H level, and thereby resets the flip-flop 6; if the error current Ie is larger, the comparator 5 turns its output signal to L (low) level so as not to reset the 6.
Here, the error current Ie from the differential amplifier 3 is a current commensurate with the error detected by comparing the feedback voltage Vadj, which is obtained by dividing the output voltage Vo with the voltage division resistors R1 and R2, with the reference voltage Vref from the reference voltage source 4. This error current Ie is compared with the switch current Isw to perform the switching of the switching device 2 so that the output voltage Vo is kept at a constant voltage commensurate with the reference voltage Vref. In this switching power supply apparatus of the current-control type, to ensure stable operation, fluctuations in the switch current Isw need to be made to converge. To achieve this, as shown in FIG. 7D, the slope of the compensated current Icc1 needs to be made steeper than the slope of the broken lines. For this purpose, the current compensation circuit 13 adds the compensation current, which increases with time, to the detection current Isens to make the slope of the compensated current Icc1 steeper.
Through this sequence of operations, even when the load 9 varies, the output voltage Vo is stably kept constant. For example, when the load 9 becomes higher, through the following sequence of operations, the output voltage Vo is stabilized. As the load 9 increases, the output voltage Vo decreases, then the error current Ie increases, and then the switch current Isw through the switching device 2 increases (with the duty kept constant) so that the peak value of the compensated current Icc1 increases (with the slope kept constant) and that the peak value of the detection current Isens increases (with the slope kept constant).
In this way, the switch current Isw increases with the load. To prevent the switch current Isw from becoming excessively large, the overcurrent detection circuit 12 performs overcurrent protection operation. As shown in FIG. 7F, the overcurrent detection circuit 12 compares the detection current Isens from the current detection circuit 11 with the set current Iocp that is previously set so as to correspond to the overcurrent detection level, and detects whether or not the switch current Isw is an overcurrent or not.
Specifically, when the detection current Isens becomes larger than the set current Iocp, the switch current Isw is judged to be an overcurrent, and thus the flip-flop 6 is fed with a reset signal so as to be reset. As a result, the switching device 2 is turned and kept off until the flip-flop 6 is set next time by the pulse signal from the oscillator 7. This limits the peak of the switch current Isw flowing through the switching device 2 (FIG. 7E), and thereby prevents an overcurrent from flowing through the switching device 2.
Japanese Patent Application Laid-Open No. H11-332222 discloses a DC/DC converter that includes a capacitor for driving a switch provided between input power and a load wherein the switch is kept closed continuously so long as the capacitor is not filly discharged. In this DC/DC converter, a coil is provided that is connected to the switch, and the duty of the switch is determined according to the difference between a detection signal obtained by detecting the current flowing through the coil and the output voltage or a reference voltage that is proportional to the output voltage. Moreover, two ramp currents are added to the detection signal to adjust the off-state period of the switch for the purpose of preventing the capacitor from being insufficiently charged.
Japanese Patent Application Laid-Open No. 2000-32744 discloses a DC/DC converter that includes a switch through which power is fed and an inductor provided between the switch and an output terminal wherein the switch is switched at regular time intervals irrespective of the current flowing through the inductor. In this DC/DC converter, when the current flowing through the inductor reaches a command value signal that is determined according to the output voltage, a flip-flop is reset so that the switch is turned off.
The conventional switching power supply apparatus shown in FIG. 6, however, has the following disadvantage. In the normal condition, the timing with which the switching device 2 is turned off is determined by comparing the compensated current Icc1, which is the sum of the detection current Isens from the current detection circuit 11 and the compensation current from the current compensation circuit 13, with the error current Ie from the differential amplifier 3. By contrast, in the overcurrent-protection-enabled condition, in which the overcurrent detection circuit 12 operates, the timing with which the switching device 2 is turned off is determined by comparing the detection current Isens from the current detection circuit 11 with the set current Iocp set in the overcurrent detection circuit 12. Thus, in this condition, the current compensation circuit 13 does not operate. As a result, when the switching operation is performed with a high duty, low-frequency oscillation occurs, causing the switching frequency to drop to a fraction of its normal frequency.
Now, the problems that arise when such low-frequency oscillation occurs will be described with reference to FIG. 8. FIG. 8 is a diagram showing the Vo−Io characteristic, i.e., the output voltage Vo plotted against the load current Io, as observed in the overcurrent-protection-enabled condition. In FIG. 8, the vertical axis represents voltage and the horizontal axis represents current, with broken lines indicating the characteristic in the normal condition and solid lines indicating the characteristic in the low-frequency-oscillation condition. Consider, for example, a case where the input voltage Vin equals 20 V, the output voltage Vo equals 15 V, the inductance L of the coil L1 equals 10 μH, the oscillation frequency fo of the oscillator 7 equals 1 MHz, an the overcurrent detection level Ic1 of the overcurrent detection circuit 12 equals 1.3 A.
The relationship between the load current Io and the switch current Isw is given byIo=Isw−(Vin−Vo)/(2Lfo)×Vo/VinSince Isw equals Ic1, the maximum value of the load current Io, i.e., the load current Io at which overcurrent protection is enabled, equals 1.11 A in the normal condition. By contrast, when low-frequency oscillation occurs and the switching frequency drops to 250 kHz, 250 kHz is substituted as fo in the above formula, which thus gives the maximum load current as 0.55 A.
Here, assume that the switching power supply apparatus is operating at a load current Io of 1 A. So long as low-frequency oscillation does not occur, even if a transient overcurrent, for example a charge current to the output capacitor C2 at start-up, appears, since there is only one operating point, namely point A, at which Io equals 1 A in FIG. 8, as soon as the overcurrent condition disappears, 15 V is outputted as the output voltage. By contrast, in a condition where low-frequency oscillation may occur, there are two operating points, namely points A and B, where Io equals 1 A, and, if the switching power supply apparatus happens to operate at point B, a voltage lower than 15 V is outputted as the output voltage.
To avoid this inconvenience, the possibility of the switching frequency dropping to ¼ of its normal frequency needs to be taken into consideration by giving the coil four times the inductance otherwise necessary. This, however, leads to increased coil size and increased cost, and thus makes it impossible to make the switching power supply apparatus compact and inexpensive.
According to the conventional technique disclosed in Japanese Patent Application Laid-Open No. H11-332222 mentioned above, even when the difference between the input voltage fed in and the output voltage to be maintained is small, it is indeed possible to maintain the output voltage, but there is provided no overcurrent protection function for protecting the switch that performs switching. Thus, when a current larger than the current capacity of the switch continues to flow therethrough, as when a high load is connected, the switch may break down.
Japanese Patent Application Laid-Open No. 2000-32744 mentioned above also discloses additionally providing a circuit for forcibly resetting the flip-flop for driving the switch on detection of an abnormality (overcurrent) in the load current. Providing such a circuit indeed helps achieve overcurrent protection, but does not solve the problem of the switching frequency dropping as a result of low-frequency oscillation occurring when overcurrent protection is enabled.